Various hydrocarbon feedstocks such as crude petroleum oils, topped crudes, heavy vacuum gas oils, shale oils, tar sand bitumens, and other heavy hydrocarbon fractions such as residual fractions and distillates contain varying amounts of non-metallic and metallic impurities. The non-metallic impurities include nitrogen, sulfur, and oxygen and these exist in the form of various compounds and are often in relatively large quantities. The most common metallic impurities include iron, nickel, and vanadium. Other metallic impurities including copper, zinc, and sodium are often found in various hydrocarbon feedstocks and in widely varying amounts. Some of the metal contaminants are present in the form of relatively thermally stable organo-metallic complexes such as metal porphyrins.
Residual petroleum oil fractions produced by atmospheric or vacuum distillation of crude petroleum are characterized by relatively high metals and sulfur content. This occurs because substantially all of the metals present in the original crude remain in the residual fraction, and a disproportionate amount of sulfur in the original crude oil also remains in that fraction.
The high metals content of the residual fractions generally preclude their effective use as charge stocks for subsequent catalytic processing such as catalytic cracking and hydrocracking, because the metal contaminants deposit on the special catalysts for these processes and cause the formation of inordinate amounts of coke, dry gas and hydrogen.
It is current practice to upgrade certain residual fractions by a pyrolytic operation known as coking. In this operation the residuum is destructively distilled to produce distillates of low metals content and leave behind a solid coke fraction that contains most of the metals. Coking is typically carried out in a reactor or drum operated at about 800.degree.-1100.degree. F. (426.degree.-593.degree. C.) temperature and a pressure of 1-10 atmospheres. The economic value of the coke byproduct is determined by its quality, particularly its sulfur and metals content. Excessively high levels of these contaminants makes the coke useful only as low-valued fuel. In contrast, cokes of low metals content, for example up to about 100 ppm (parts per million by weight) of nickel and vanadium, and containing less than about 2 weight percent sulfur may be used in high-valued metallurgical, electrical, and mechanical applications.
Certain residual fractions are currently subjected to visbreaking, which is a heat treatment of milder conditions than used in coking, in order to reduce their viscosity and make them more suitable as fuels. It usually involves a short soak time at a temperature of about 800.degree.-950.degree. F. (443.degree.-510.degree. C.) The mild thermal cracking conditions of visbreaking produces about 5-15 percent of gas oil, about 5-15 percent of gasoline, and about 70-85 percent of heavy fuel oil. Typical visbreaking procedures are described in U.S. Pat. Nos. 2,358,573 and 2,695,264, and in 1980 Refining Process Handbook, page 158 (Reprinted from September 1980 issue of Hydrocarbon Processing, Gulf Publishing Co., Houston, Texas).
Visbreaking does not significantly reduce the metals content of the visbroken distillate fractions. For example, the gas oil from the visbroken effluent contains at least about 1-25 ppm of nickel and vanadium.
The economic and environmental factors relating to upgrading of petroleum residual oils and other heavy hydrocarbon feedstocks have encouraged efforts to provide improved processing technology, as exemplified by the disclosures of various United States patents such as U.S. Pat. Nos. 2,591,525; 2,717,865; 2,761,816; 2,909,476; 2,921,022; 2,950,231; 2,987,470; 3,094,480, 3,146,188; 3,594,312; 3,663,434; 3,676,369; 3,696,027; 3,716,479; 3,766,054; 3,772,185; 3,775,303; 3,813,331; 3,839,187; 3,847,798; 3,876,530; 3,882,049; 3,897,329; 3,901,792; 4,062,757; and the like, and references cited therein.
There is continuing research effort to improve the efficiency of processing means for upgrading of hydrocarbon feedstocks, with particular reference to petroleum residual oils.
Accordingly, it is an object of this invention to provide an improved process for converting heavy hydrocarbon oils into effluent fractions having a substantially reduced content of sulfur, metal and nitrogen contaminants.
It is another object of this invention to provide a process for converting heavy hydrocarbon feedstocks by thermal treatment into liquid hydrocarbon fractions which boil in the gasoline range and in the gas oil range between about 400.degree.-900.degree. F. (204.degree.-482.degree. C.), substantially without the formation of coke.
It is a further object of this invention to provide a process for upgrading heavy hydrocarbon oils into gasoline and gas oil fractions, and into a residual tar fraction which has a fusion temperature below about 160.degree. C.
Other objects and advantages of the present invention shall become apparent from the accompanying description and illustrated Example.